The Expect value (E) is a parameter that describes the number of hits one can "expect" to see just by chance when searching a database of a particular size. It decreases exponentially with the Score (S) that is assigned to a match between two sequences. Essentially, the E value describes the random background noise that exists for matches between sequences. For example, an E value of 1 assigned to a hit can be interpreted as meaning that in a database of the current size one might expect to see 1 match with a similar score simply by chance. This means that the lower the E-value, or the closer it is to "0" the more "significant" the match is. However, keep in mind that searches with short sequences, can be virtually indentical and have relatively high EValue. This is because the calculation of the E-value also takes into account the length of the Query sequence. This is because shorter sequences have a high probability of occuring in the database purely by chance. For more details please see the calculations in the BLAST Course.
The Expect value can also be used as a convenient way to create a significance threshold for reporting results. You can change the Expect value threshold on most main BLAST search pages. When the Expect value is increased from the default value of 10, a larger list with more low-scoring hits can be reported.
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BLAST 2 SEQUENCES RESULTS VERSION BLASTP 2.2.4 [Aug-26-2002] Matrix gap open: gap extension:
x_dropoff: expect: wordsize: Filter
| Sequence 1 | gi 15606306 | flagellar motor protein MotA [Aquifex aeolicus] | Length | 254 | (1 .. 254) |
| Sequence 2 | gi 21399546 | MotA_ExbB, MotA/TolQ/ExbB proton channel family [Bacillus anthracis A2012] | Length | 254 | (1 .. 254) |
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2 | 1 |
NOTE:The statistics (bitscore and expect value) is calculated based on the size of nr database
Score = 138 bits (348), Expect = 9e-32
Identities = 82/237 (34%), Positives = 136/237 (56%), Gaps = 6/237 (2%)
Query: 7 IGIIAAFLLILISILIGG----SITAFINVPSIFIVVGGGMAAAMGAFPLKDFIRGVLAI 62 +GII F +++ +I++GG + F++V SI IV+GG A + A+ + + +I Sbjct: 1 MGIIVGFAIVIAAIMLGGGGIKAFKNFLDVSSILIVIGGTTATIVVAYRFGEIKKYTKSI 60 Query: 63 KKAFLWKPPDLNDVIETIGEIASKVRKEGILALEGDIELYYQKDPLLGDMIRMLVDGIDI 122 + DL + + + + K +K G+L+LE D E +P + IR+++ G D Sbjct: 61 FTVLHRREEDLEQLTDLFVDFSKKSKKHGLLSLEVDGEQV--DNPFIQKGIRLMLSGYDE 118 Query: 123 NDIKATAEMALAQLDEKMSTEVAVWEKLADLFPAFGMIGTLIGLIQMLRNLNDPSALGPG 182 ++K + ++ A+ +K+ D PA+GMIGTLIGLI ML+NL D S +G G Sbjct: 119 EELKEVLMKDVETEVYELRKGAALLDKIGDFAPAWGMIGTLIGLIIMLQNLQDTSQIGTG 178 Query: 183 MAVALITTLYGAILANAFAIPVANKLKKAKDMEVLVKTIYIEAIEKIQKGENPNVVK 239 MAVA++TTLYG++LAN AIP+A K+ + + K IEAI ++ +G+ P+ +K Sbjct: 179 MAVAMLTTLYGSVLANMIAIPLAEKVYRGIEDLYTEKKFVIEAISELYRGQIPSKLK 235
CPU time: 4.58 user secs. 0.58 sys. secs 5.16 total secs. Lambda K H 0.321 0.141 0.383 Gapped Lambda K H 0.267 0.0410 0.140 Matrix: BLOSUM62 Gap Penalties: Existence: 11, Extension: 1 Number of Hits to DB: 604 Number of Sequences: 0 Number of extensions: 59 Number of successful extensions: 3 Number of sequences better than 300.0: 1 Number of HSP's better than 300.0 without gapping: 1 Number of HSP's successfully gapped in prelim test: 0 Number of HSP's that attempted gapping in prelim test: 0 Number of HSP's gapped (non-prelim): 1 length of query: 254 length of database: 396,279,676 effective HSP length: 123 effective length of query: 131 effective length of database: 396,279,553 effective search space: 51912621443 effective search space used: 51912621443 T: 9 A: 40 X1: 16 ( 7.4 bits) X2: 129 (49.7 bits) X3: 129 (49.7 bits) S1: 41 (21.8 bits) S2: 60 (27.7 bits)